FileSpec: Remove PathSyntax enum and use llvm version instead

Summary:
The llvm version of the enum has the same enumerators, with stlightly
different names, so this is mostly just a search&replace exercise. One
concrete benefit of this is that we can remove the function for
converting between the two enums.

To avoid typing llvm::sys::path::Style::windows everywhere I import the
enum into the FileSpec class, so it can be referenced as
FileSpec::Style::windows.

Reviewers: zturner, clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D46753

llvm-svn: 332247